The present invention relates to data transmission systems and, more particularly, to the identification of crosstalk interference in a communication system.
Digital subscriber line (DSL) technology uses the existing telephone twisted pairs to provide high-speed internet access services to both residential and business customers. There are many types of DSLs, which are generically referred to as xDSL, including basic rate DSL (ISDN), high-bit-rate DSL (HDSL), second generation HDSL (HDSL2), asymmetric DSL (ADSL), symmetrical DSL (SDSL), and very-high-bit-rate DSL (VDSL). Today in the United States, several million telephone lines between central offices and subscribers are deployed with xDSL technology, and the number of the subscribers is rising rapidly.
These wide band modulation approaches present inherent obstacles that must be overcome. One particular problem relates to crosstalk interference that is introduced to the twisted pair transmission line and received by the modem. As is well known to those skilled in the art, crosstalk interference is unwanted interference (signal noise) that is passed between adjacent network cables or devices. Crosstalk generally occurs due to coupling between wire pairs when wire pairs in the same or a nearby bundle are used for separate signal transmission. In this manner, data signals from one or more sources may be superimposed on and contaminate a data signal from a second source. The crosstalk includes near-end crosstalk (NEXT) and far-end crosstalk (FEXT). In ADSL and VDSL systems, frequency-division duplexing can be used to avoid NEXT. Nevertheless, NEXT may still exist because of other types of services like ISDN, HDSL, HDSL2, SDSL and T1.
As can be appreciated, the data signals being transmitted over the twisted-pair phone lines can be significantly degraded by the crosstalk interference generated on one or more adjacent twisted-pair phone lines in the same and/or a nearby bundle. As the speed of the data transmission increases, the problem worsens. For example, in the case of VDSL signals being transmitted over the twisted-pair phone lines, the crosstalk interference can cause significant degradation of the VDSL signals, including substantially limiting the maximum data rate of an individual line. To prevent a breakdown of currently deployed systems, operators frequently assume and compensate for the worst case scenario (that is, the highest level of crosstalk interference). However, this assumption is often too pessimistic when compared to actual crosstalk interference on the transmission line and hence unnecessarily limits the overall performance of the system. If actual crosstalk interference could be identified, then the crosstalk could either be removed (or lessened) or the system could be operated in a manner that does not unnecessarily compensate for a level of crosstalk that is not present.
Identification of crosstalk coupling functions within telephone lines can yield several overwhelming benefits. First, the crosstalk functions can be used in a multi-user detector in a line's modem to cancel the strong interference from other lines. Second, it can improve the data rate (or the line reach scope) of a system by better spectrum management, such as a better spectrum assignment for different users. For example, if one user causes strong crosstalk to another user in a particular frequency band, the modem may be switched to avoid transmitting in this frequency band in lieu of a multi-user detector. Third, crosstalk profiles are invaluable for the telephone operators to maintain, diagnose, and expand the current systems.
However, it has proven to be exceedingly difficult to identify crosstalk functions among copper wires because lines in the same bundle could belong to different service operators as a result of the unbundling process and regulatory action undertaken in many parts of the world. For example, in the United States and some other countries, competitive local exchange carriers (CLECs) can lease the telephone lines from incumbent local exchange carriers (ILECs, the traditional phone companies) and offer xDSL services to the local subscribers. As a result, the modems from different operators are asynchronous. Even within the same service operator, different types of services (HDSL, ADSL, ISDN, etc.) are offered in the same bundle and these services have different symbol rates.
Because the modems in the same bundle could belong to different service operators (CLECs and ILECs), the time stamps of the data from different operators' modems can be offset by several milliseconds. Therefore, an additional problem with identifying crosstalk interference with existing xDSL systems is the presence of timing differences between the transmitted data from different users and the received data from one designated receiver. Currently, the timing difference between two signals can be greater than one thousand data symbols.
Moreover, in the multi-operator environment, spectral compatibility among the different operators is a major concern. Spectral compatibility is fundamentally determined by the crosstalk level caused by different users. For the foregoing reasons, some level of coordination and agreement in which all operators' interests are fairly considered and benefited would be helpful to all users of such xDSL systems.
Therefore, crosstalk problems arising from using twisted-pair phone lines with high data Transmission rates, including ADSL and VDSL for example, become a substantial impediment to a receiver being able to properly receive the transmitted data signals. Thus, there is a need to provide techniques to identify and determine the timing differences in various data signals and to identify and determine the magnitude and phase of crosstalk interference so that steps can be taken to reduce or eliminate such interference, improve line maintenance and assist in spectrum assignment.